Current vehicle head lamps, brake lamps, running lamps, turn signal lamps, fog lamps, back-up lamps and parking lamps (hereinafter referred to collectively as "lamps" or "vehicle lamps" for convenience) typically have the light bulb located in an enclosed housing of the lamp not only for aesthetic appearance, but also to prevent water, dirt, oils and the like from reaching the bulb, the reflective surfaces, and the light transmitting surfaces of the lamp. It is often the case however, that upon thermal cycling during use of the bulb, thermal cycling due to changes in the environment, or thermal cycling as a result of vehicle operation, moisture condenses on the interior of the housing and inhibits light output from the lamp.
Various venting concepts and desiccant assemblies have been used conventionally to minimize the effects of condensation build-up in enclosed lamp housings. For instance, some conventional vehicle lamps having an enclosed housing include a desiccant for preventing fog formation on the internal walls of the lamp or its reflector. The desiccant adsorbs water vapor which enters the housing when the lamp is off. When the lamp is turned on, heat generated by the bulb dries the air and the desiccant, thereby regenerating the desiccant. The desiccant is usually in the form of a housed or packaged silica gel or similar material.
Although this type of packaged desiccant provides adequate moisture adsorption under some conditions and is capable of being regenerated by the heat produced by the light bulb, a number of difficulties have been identified with these types of systems. For example, the desiccant package is not easy to position within the housing, often requiring that a special sub-housing be provided within the lamp housing. In addition, this type of packaged desiccant subhousing cannot withstand the high temperatures generated by some light bulbs, and accordingly, the desiccant must be located at least a minimum distance from a high temperature bulb and/or shielded from the bulb.
Moreover, desiccant assemblies can add significant costs to lighting systems. New approaches which can reduce part cost and complexity are constantly being sought by manufacturers of vehicle lamps.
Vent systems that reduce condensation often employ some means of increasing airflow through the lamp housing. In general, the atmospheric air outside of a lamp housing is below the water vapor saturation point, and the air flowing through the housing has the capacity to remove condensation from the lamp housing by removing water vapor from the housing. Vent systems using this means of condensation reduction generally have vent openings in more than one location. The openings are often placed in locations where air flow past the vent opening enhances air flow through the vent openings. The location of these vent systems can be an important consideration. However, such vent systems that provide a means of increasing air flow through the lamp housing often have a negative effect on lamp performance. Particularly, these venting systems often create an opportunity for foreign materials and liquid water to enter the vehicle lighting system.
Vents have also been used within closed housings to relieve pressure build-up due to changes in environmental conditions (e.g., when the bulb(s) are energized, changes in outside temperature, etc.) while minimizing the entry of water and dirt into closed lamp housings. For example, vents that incorporate microporous materials such as expanded PTFE membranes (e.g., GORETEX.RTM. membrane vents, available from W. L. Gore and Associates, Inc., Elkton, Md.), modified acrylic copolymer membranes (VERSAPORE.RTM. membranes, available from Gelman Sciences, Ann Arbor, Mich.), and other microporous materials are commonly used to relieve pressure from lamps and have proven to be very effective means of preventing liquid water entry and entry of foreign materials in the lamp housings. As used herein, the term "microporous material" is intended to refer to a continuous sheet of material that is at least 25% porous (i.e., it has a pore volume of .gtoreq.25%), with 50% or more of the pores being no more than about 30 micrometer in nominal diameter.
Microporous materials are sold in many configurations. For example, microporous materials are available with plastic housings that protect the material from damage and contamination, while simplifying installation. Some microporous materials are supplied with woven and/or nonwoven fabrics that provide protection for the microporous material. Microporous materials with or without fabric support have been made into products that incorporate adhesives for the purpose of attaching the product to a device that is vented.
Conventional microporous vent products designed for vehicle lighting applications have addressed pressure relief, ease of installation, durability, exclusion of liquid water and foreign materials, etc. The conventional design requirements for the microporous vent area are based on maintaining low pressures within the lamp housing during thermal cycling of the lamp. The venting surface area of microporous vents is designed based on the air flow of the microporous vent material and the volume change of the air in the lamp resulting from thermal cycling.
A significant concern regarding the use of microporous vents is that the venting configurations available do not effectively remove condensation. It is common to find references in existing art that recommend small vent sizes for porous vent materials. One example is U.S. Pat. No. 4,802,068, in the name of Mokry, which teaches that "[t]he size of the opening and the composition of the element are selected to permit adequate variation in the air pressure within the chamber. The opening should not be too large however, or the rate of transmission of moisture through the seal may be unacceptably high. Conveniently the opening may be provided by a hole about 5 mm in diameter." (col. 3, lines 27-33) The microporous product designs are tested for condensation performance by exposing the lamp and vent assemblies to various temperature and humidity combinations. These tests are used to determine how well the microporous vent products will perform with respect to condensation formation and elimination.
However, it has not been taught that condensation within, on or integral with the housing can be reduced, and preferably eliminated, by providing a condensation vent comprising a water vapor permeable material of specific surface area and specific dimensions and compositions. As used herein, the term "water vapor permeable" means a material or system which permits the passage of water vapor through the material system.
Thus, to date, there has not been a satisfactory system for reducing condensation in enclosed vehicle lamp housings which combines the benefits not only of eliminating or reducing entry of liquid water and other foreign materials, but also of minimizing or eliminating the formation of condensation within a lamp housing.
Accordingly, there has been a long-felt need in the art for a system for rapidly removing and reducing the build-up of condensation in vehicle lamps.